Holding the Command key (Mac) or Control key (Windows) should allow for selecting/highlighting multiple, separate characters or words within a text field. Similar to how the Shift key can be used to select/highlight a single range of characters.

This would allow easier batch modification, to be used when making many singular characters within a paragraph into subscripts, for instance. Command or Control-selecting each character to be modified then enables the user to only have to click the subscript button once, instead of for each character after highlighting.